Method and unit for separating the light and heavy components of natural gas

Abstract

The invention relates to a method for the treatment of natural gas containing carbon dioxide, methane and paraffins. The method comprising: a step of extracting the paraffins from the natural gas in a paraffin-removal column, and a step of separating the carbon dioxide and the methane in a distillation column. The operation of the two columns being provided by means of the thermal coupling of said two columns using a thermal coupling heat exchanger.

The invention claimed is: 
     
       1. A process for treating a natural gas comprising carbon dioxide, methane and paraffins, so as to obtain a gas stream enriched with methane, a liquid stream enriched with carbon dioxide and a liquid stream enriched with paraffins, said process comprising the following steps of:
 a) introducing the natural gas into a deparaffinizing column, 
 b) extracting the paraffins from the natural gas in the deparaffinizing column, and recovering a top vapour stream comprising carbon dioxide and methane, at the column top, and said liquid stream enriched with paraffins at the column bottom, 
 c) cooling the top vapour stream then introducing it into a separation unit, 
 d) recovering a liquid stream from the separation unit and a vapour stream from the separation unit, 
 e) introducing all or part of the liquid stream from the separation unit, as reflux liquid, at the top of the deparaffinizing column so as to wash, in countercurrent mode, the natural gas introduced in step a), 
 f) cooling then expanding the vapour stream from the separation unit recovered in step d) so as to obtain a condensed stream and a first gas stream enriched with methane M 1 , 
 g) introducing the condensed stream into a distillation column so as to recover, at the top of the column, a second gas stream enriched with methane M 2  and, at the column bottom, said liquid stream enriched with carbon dioxide, 
 h) mixing the first gas stream enriched with methane M 1  and the second gas stream enriched with methane M 2  so as to obtain said gas stream enriched with methane, 
 i) reheating a part of the liquid stream enriched with carbon dioxide and introducing the resulting reheated stream into the distillation column, 
 
       in which the cooling of the top vapour stream in step c) and the reheating of the part of the liquid stream enriched with carbon dioxide in step i) are carried out by passing said top vapour stream and said part of the liquid stream enriched with carbon dioxide through a thermal coupling heat exchanger. 
     
     
       2. The process according to  claim 1 , wherein a part of the liquid stream recovered in step d) is withdrawn and injected, as feed liquid, into the distillation column. 
     
     
       3. The process according to  claim 1 , wherein the cooling of step c) is carried out in several successive cooling operations before the introduction into the separation unit ( 5 ). 
     
     
       4. The process according to  claim 1 , wherein the gas stream enriched with methane is heated then compressed before being further mixed with a gas stream from a first methane gas recovery line. 
     
     
       5. The process according to  claim 1 , wherein a part of the liquid stream enriched with carbon dioxide is injected into a geological reservoir. 
     
     
       6. The process according to  claim 1 , wherein the natural gas comprises from 35% to 85% of carbon dioxide, from 15% to 65% of methane and at most 4% of paraffins, the percentages being given in mole values. 
     
     
       7. The process according to  claim 1 , wherein the gas stream enriched with methane comprises less than 22% of carbon dioxide, the percentages being given in mole values. 
     
     
       8. The process according to  claim 1 , wherein the liquid stream enriched with carbon dioxide comprises from 60% to 100% of carbon dioxide, the percentages being given in mole values. 
     
     
       9. The process according to  claim 1 , wherein the liquid stream enriched with paraffins comprises from 4% to 40% of paraffins, the percentages being given in mole values. 
     
     
       10. An apparatus for treating a natural gas comprising carbon dioxide, methane and paraffins, so as to obtain a gas stream enriched with methane, a liquid stream enriched with carbon dioxide and a liquid stream enriched with paraffins, wherein said apparatus comprises:
 a deparaffinizing column, 
 an inlet line connected at the inlet of the deparaffinizing column so as to introduce the natural gas into the deparaffinizing column, 
 a deparaffinizing column top line connected at the top of the deparaffinizing column so as to recover a top vapour stream and a deparaffinizing column bottom line connected at the bottom of the deparaffinizing column so as to recover a liquid stream enriched with paraffins, 
 a separation unit fed by the deparaffinizing column top line, 
 a reflux line connected to the separation unit so as to recover and reinject a liquid stream, as reflux liquid, as the top of the deparaffinizing column, 
 a vapour line connected to the separation unit so as to recover a vapour stream, 
 a heat exchanger and pressure reducing valves/separators fed by the vapour line so as to obtain a condensed stream recovered in a condensate recovery line and a first gas stream enriched with methane M 1  recovered in a methane recovery line, 
 a distillation column fed by the condensate recovery line, 
 a distillation column top line connected at the top of the distillation column so as to recover a second gas stream enriched with methane M 2 , 
 a methane collection line connected to the distillation column top line and the methane recovery line so as to recover a gas stream enriched with methane, 
 a distillation column bottom line connected at the bottom of the distillation column so as to recover a liquid stream enriched with carbon dioxide, 
 a drawing off line connected to the distillation column bottom line so as to draw off and reinject a part of the liquid stream enriched with carbon dioxide into the distillation column, 
 
       wherein said apparatus also comprises a thermal coupling heat exchanger through which pass the deparaffinizing column top line and the drawing off line configured to cool the top vapour stream before introduction thereof into the separation unit while at the same time reheating the drawing-off part of the liquid stream enriched with carbon dioxide before injection thereof into the distillation column. 
     
     
       11. The apparatus according to  claim 10 , wherein an injection line is connected to the reflux line for drawing off a part of the reflux liquid stream and injecting it, as feed liquid, into the distillation column. 
     
     
       12. The apparatus according to  claim 10 , wherein a refrigeration unit and the vapour line, the methane collection line, and a first methane recovery line passes through the heat exchanger so as to cool the vapour stream while at the same time reheating the gas stream enriched with methane and the first gas stream enriched with methane M 1 . 
     
     
       13. The apparatus according to  claim 10 , wherein the deparaffinizing column top line passes through the heat exchanger so as to cool the top vapour stream after passing through the thermal coupling heat exchanger and before it is introduced into the separation unit. 
     
     
       14. The apparatus according to  claim 10 , wherein the deparaffinizing column bottom line feeds the distillation column.